1. Field of the Invention
The present invention relates to an apparatus for reproducing a digital signal recorded on a recording medium.
2. Description of the Related Art
There is known a digital signal reproducing apparatus which compares the level of a signal read from a recording medium with a predetermined slice level, and reproduces digital data of "1" and "0" with this predetermined slice level taken as a threshold value.
FIG. 1 shows the structure of this digital signal reproducing apparatus, and FIG. 2 illustrates one example of the operation of that structure.
First, a read signal (a) read from a recording medium is supplied to a comparator 1. The comparator 1 compares the level of the read signal (a) with a slice level (b). When the level of the read signal (a) is equal to or higher than the slice level (b), the comparator 1 supplies a binary signal (c) of a high level "1" to a determining circuit 2 and a clock generator 3. When the level of the read signal (a) is lower than the slice level (b), the comparator 1 supplies a binary signal (c) of a low level "0" to the determining circuit 2 and the clock generator 3. The clock generator 3 generates a clock (d) whose phase is in synchronous with the edge timing of this binary signal (c). The determining circuit 2 samples the binary signal (c), supplied from the comparator 1, at the timing of the clock (d) from the clock generator 3, yielding a digital data output (e).
As apparent from the above, the digital signal reproducing apparatus is designed in such a way that the phase of the sampling clock matches with the cross point between the read signal and the predetermined slice level. Thus, the phase of the sampling clock synchronizes with the timing of the middle point in the level transition at the time the read signal changes its level from "1" to "0" or from "0" to "1", i.e., with the timing at which the level of the read signal becomes the center level of the maximum amplitude the read signal itself has. It is therefore possible to sample the read signal at the accurate timing.
In a reproducing apparatus that employs a partial response system in which the read signal is a multi-value signal that will take three values, five values, etc., the read signal does not vary near the center level of the maximum amplitude and becomes a flat waveform in some case. In such a case, however, the read signal and the predetermined slice level would have a number of cross points, so that the phase of the sampling clock cannot be compensated accurately.